Exercise, Memory Management

Tom Kelliher, CS 320

Jan. 29, 2003

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Assignment

Read 1.5--9.

From Last Time

I/O in C.

Outline

1. I/O exercise.

2. Structures and pointers in C.

Coming Up

Memory allocation in C.

Exercise

Complete the exercise from last time. You did sketch out the code, didn't you?

Structures

1. Public classes without methods.

2. General structure:

   struct <struct_identifier>
   {
      <member_declaration>
      [<member_declaration> ...]
   };
   /* Don't forget the semicolon!!! */
   

3. Example:

   #include <stdio.h>
   
   /* "struct dimension" becomes a new type. */
   
      struct dimension
      {
         double length;
         double width;
         double height;
      };
   
   
   /* Prototypes */
   
   void printDimension(struct dimension);
   
   
   int main()
   {
   
      struct dimension box1 = { 1.0, 1.0, 1.0 };
      struct dimension box2;
   
      box2.length = 2.0;
      box2.width = 4.0;
      box2.height = 6.0;
   
      printDimension(box2);
   
      return 0;
   }
   
   
   void printDimension(struct dimension dim)
   {
      printf("Length: %g\nWidth: %g\nHeight: %g\n", dim.length,
             dim.width, dim.height);
   }
   

Pointers

1. Pointer variables hold the address of another variable.

2. Examples similar to what we've already seen:

   double data[10];
   double *p_data;
   int sum;
   int *p_sum;
   
   p_data = data;
   p_data[3] = 0.0;
   
   p_sum = ∑
   sum = 10;
   printf("Sum: %d\n", p_sum);
   
   *p_sum = 12;   /* Dereference the pointer */;
   

3. What's going on here?

   

4. Pointer arithmetic
   1. You can never add two pointers, but you can add a pointer and an integer:

      double sum;
      double data[10];
      double *dp;
      int i;
      
      sum = 0.0;
      for (i = 0, dp = data; i < 10; i++, dp++)
         sum += *dp;
      

      Note that dp will be incremented by sizeof(double).

   2. data[4] is another way of writing *(data + 4).

   3. You can subtract two pointers:

      int strlen(char *s)
      {
         char *ptr = s;
      
         while (*ptr != '\0')
            ptr++;
      
         return ptr - s;
      }
      

5. Exercise: Plain vanilla C arrays always start at an index of 0. Using what we just learned, how could you use an array and a pointer variable to create an array which began at a negative index?